~600RPM in Drive???

okay, I'm not sure if this is new or not, but it just piqued my curiousity and my paranoia..

if I'm driving along at whatever speed, and I'm not going uphill or anything, and i let off the gas to sort of coast, it seems like the car goes into neutral and the revs drop down well below 1000. There's no noticable "snap" when I get back on the gas or anything.. is this normal?

-adam